1. Field of the Invention
The present invention relates generally to improvements to antennas, and more particularly to advantageous aspects of a microstrip patch antenna with an embedded impedance transformer.
2. Description of the Prior Art
In a typical microstrip patch antenna, the radiator element is provided by a metallic patch that is fabricated onto a dielectric substrate over a ground plane. Microstrip patch antennas play an important role in the antenna field because of their many desirable features. These include their low profile, reduced weight, relatively low manufacturing cost, polarization diversity and a relatively easy integration process that allows many identical patches to be grouped into arrays and to be integrated with circuit elements.
In order to function efficiently, an antenna""s input impedance should match that of its transmission feed line. Various techniques are used to accomplish impedance matching in a microstrip patch antenna. In a patch antenna employing a coaxial feed, illustrated in FIG. 3 and described below, impedance matching is typically accomplished by adjusting the position of the patch element feed point. However, as discussed below, the range of impedance matching available using this approach is limited by the physical dimensions of the patch element.
Although it would be theoretically possible to obtain the desired impedance matching by varying the design parameters of the patch antenna other than the size of the patch element, this variation is often not practical. The input impedance of a microstrip patch antenna is determined by a number of factors, including the dimensions of the patch, the height of the substrate, and by dielectric parameters. However, there can be relatively limited flexibility in the adjustment of these factors. For example, the dielectric loading of the antenna as well as the patch dimensions may be dictated by the required beamwidth and resonance characters for the antenna.
The prior art can be better understood with reference to FIGS. 1 through 3, which illustrate three basic techniques that are currently used to feed a microstrip antenna. These include, respectively, transmission line feed, aperture feed, and coaxial feed.
FIG. 1 shows a perspective view of a patch antenna 10 employing a transmission line feed technique. As shown in FIG. 1, antenna 10 includes a substantially square patch element 12 that has been fabricated onto a dielectric substrate 14 lying on top of a ground plane 16. The feed line 18 to the patch element 12 has been fabricated onto the same substrate 14 as the patch element 12 and directly connects to an edge of the patch element 12, with an inset 20 cut into the patch 12. The transmission line feed is a very simple way to feed a microstrip patch. Impedance matching is accomplished by adjusting the dimensions of the inset 20.
The transmission line feed approach suffers from several problems. First, since the feed line and the patch element are on the same level, they cannot be optimized simultaneously. Second, the feed line in this structure functions as another radiator, which generates spurious radiation and results in degradation of cross-polarization discrimination and pattern performance. In addition, in order to control the radiation from the feed line, the line width cannot be too wide, which results in a relatively thin substrate. It is known that, in general, the bandwidth of a microstrip antenna is proportional to the thickness of the substrate. Therefore, this type of feed leads to a narrow bandwidth structure.
FIG. 2 shows a partial cutaway perspective view of a patch antenna 30 utilizing the aperture feed approach. The antenna 30 includes a patch element 32 that has been fabricated onto a first dielectric substrate 34 lying on top of a ground plane 36. A microstrip feed line 38 is fabricated onto the bottom surface of a second dielectric substrate 40 lying underneath the ground plane 36. Coupling between the microstrip feed line 38 and the patch element 32 is accomplished by a slot 42 in the ground plane 40 that lies across the microstrip feed line 38. Finally, a metal plate reflector 44 is typically provided underneath the other antenna elements to reduce spurious radiation from the slot opening 42 in the ground plane 36.
The aperture feed approach rectifies several drawbacks associated with the transmission line feed approach, including the spurious radiation from the microstrip feed line and fundamental bandwidth limitations because the microstrip feed line 38 is underneath the ground plane 36 and can be designed independently. However, because of the existence of the reflector 44, it is possible for parallel modes to be easily excited and travel between the ground plane and the reflector. These parallel modes degrade the antenna radiation efficiency. Therefore, one major challenge in the aperture feed structure is how to suppress parallel modes.
FIG. 3 shows a perspective view of a patch antenna 50 employing the coaxial feed approach. The antenna 50 includes a patch element 52 fabricated on top of a dielectric substrate 54. A ground plane 56 abuts the lower surface of the dielectric substrate 52. Finally, a coaxial feed line 58 is mounted perpendicular to the lower surface of the ground plane 56. The outer conductor 60 of the coaxial feed line 58 is electrically connected to the ground plane 56, and the inner conductor 62 of the coaxial feed line 58 is electrically connected to the underside of the patch element 52. The input impedance is a function of the position of the feed 62 into the patch element 52. Thus, the impedance of the patch antenna 50 can be matched to the line by properly positioning the feed line 58. Because the coaxial feed line 58 directly carries current to the radiation element, patch 52, it provides a more stable signal coupling than the aperture feed structure. In addition, with a coaxial feed approach there is less concern regarding parallel mode excitation in those situations where a higher dielectric loading is required to achieve certain electrical performance characteristics such as a wider beamwidth.
In the coaxial feed approach illustrated in FIG. 3, the position of the feed can be critical in matching the input impedance of the patch element, particularly since other factors determining the input impedance, such as the patch dimensions, the height of the substrate, and the dielectric parameters, may be dictated by required antenna specifications, such as the antenna beamwidth and resonant frequency. However, in certain situations, it may be difficult or impossible to find a desired matched feed position within the available patch dimensions. Thus, the range of impedance matching available for a given microstrip patch antenna is limited.
The above-described issues and others are addressed by the present invention, one aspect of which provides an antenna having a patch element fabricated onto a substrate, a ground plane, and an impedance transformer between the patch element and the ground plane. The patch element electrically connected to a first end of the impedance transformer, and a feed line is electrically connected to a second end of the impedance transformer through the ground plane. The use of the impedance transformer allows impedance matching to be accomplished without being limited by the physical limitations of the patch element. According to a further aspect of the invention, a patch element is fabricated onto a first substrate surface and a ground plane is fabricated onto a second substrate surface, the ground plane separated from the patch element by a plurality of substrate layers. An impedance transformer is embedded between abutting substrate layers between the patch element and the ground plane, and an electrically conductive via connects a first end of the impedance transformer to a feed point on the patch element. The antenna further includes a coaxial feed having an outer conductor electrically connected to the ground plane and an inner conductor electrically connected to a second end of the impedance transformer, such that a signal is carried between the coaxial feed and the patch element through the impedance transformer.
Additional features and advantages of the present invention will become apparent by reference to the following detailed description and accompanying drawings.